1. Field of the Invention
This invention relates to a control system for a front-and-rear wheel drive vehicle, which drives one pair of respective pairs of front wheels and rear wheels by an engine and the other pair of the pairs by an electric motor, and connects and disconnects the other pair of the pairs to and from the electric motor by a clutch of a type actuated by pressure.
2. Description of the Prior Art
Conventionally, a control system for a front-and-rear wheel drive vehicle is disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 9-315164, which drives one pair of respective pairs of front wheels and rear wheels by an engine and drive the other pair of the pairs by an electric motor. In the front-and-rear wheel drive vehicle disclosed in this publication, the front wheels are driven by the engine, the rear wheels are driven by the electric motor, and the clutch is arranged to connect and disconnect between the rear wheels and the electric motor. In the control system, the clutch is engaged when the vehicle speed is lower than a first predetermined speed including during stoppage of the vehicle, and disengaged when the vehicle speed has reached a second predetermined speed higher than the first predetermined speed. Thus, the clutch is controlled to an engaged state when the vehicle makes a start or drives off.
Further, as the clutch for a wheel-driving system, a so-called hydraulically-actuated clutch is conventionally known, which is engaged or disengaged by supply of oil pressure from a clutch-actuating mechanism. The clutch-actuating mechanism of this kind includes a type comprised of an oil pump driven by a driving force of the wheel-driving system and an accumulator for accumulating oil pressure increased by the oil pump. In the clutch-actuating mechanism constructed as above, the clutch is engaged by the oil pressure from the accumulator when the vehicle makes a start.
When the above-mentioned the hydraulically-actuated clutch and clutch-actuating mechanism are applied to the control system for a front-and-rear wheel drive vehicle, the oil pump is driven for the hydraulically-actuated clutch, by the driving force of the wheel-driving system, and hence the pressure in the accumulator is sometimes considerably reduced if the vehicle is standing for a long time period. In such a case, when the vehicle makes a start, the clutch cannot be fully engaged only by the oil pressure from the accumulator, and further can be fully engaged only after oil pressure is supplied from the oil pump after the vehicle has traveled over a certain distance. Therefore, during the start of the vehicle, there temporarily occurs a front wheel-drive state in which the vehicle has only its front wheels driven by the engine, causing shortage of a total driving force for driving the vehicle. This makes it impossible to ensure sufficient startability of the vehicle on a road having a small surface resistance. Further, also in an operating condition in which the vehicle repeatedly switches between very low-speed travelling close to stoppage and normal travelling, the pressure in the accumulator is reduced due to repetition of engagement and disengagement of the clutch, which can eventually hinder full engagement of the clutch. In this case as well, there temporarily occurs the front wheel-drive state of the vehicle in which the vehicle is driven by the engine alone, causing the shortage of the total driving force for driving the vehicle. Especially, the front-and-rear wheel drive vehicle of this kind is in strong demand for use on roads having small surface resistances, such as snowy roads, or rough roads on which the vehicle is compelled to repeatedly switch between very low-speed travelling and normal travelling. Therefore, the incapability of ensuring sufficient startability or possibility of incurring shortage of a total driving force for driving the vehicle leads to reduced marketability of the vehicle.
It is an object of the invention to provide a control system for a front-and-rear wheel drive vehicle which connects and disconnects one pair of respective pairs of front wheels and rear wheels to and from an electric motor for driving the one pair by a clutch of a type actuated by pressure, in which the control system is capable of causing engagement of the clutch promptly and reliably when the vehicle makes a start and when the vehicle is travelling, thereby ensuring excellent startability of the vehicle and a sufficient total driving force for driving the vehicle.
To attain the above object, the present invention provides a control system for a front-and-rear wheel drive vehicle that drives one pair of respective pairs of front wheels and rear wheels by an engine and the other pair of the pairs by an electric motor via a clutch of a type actuated by pressure, and drives the clutch by accumulator pressure increased by a pump and accumulated in an accumulator to connect and disconnect the other pair to and from and the electric motor.
The control system according to the present invention is characterized by comprising:
pressure-detecting means for detecting the accumulator pressure;
clutch-actuating means for selectively engaging and disengaging the clutch by supplying the accumulator pressure to the clutch;
pump-driving means for driving the pump; and
control means for causing the pump-driving means to drive the pump to thereby increase the accumulator pressure when the detected accumulator pressure is equal to or lower than a predetermined value.
According to this control system, when the accumulator pressure is equal to or lower than the predetermined value, the control means causes the pump-driving means to drive the pump to thereby increase the accumulator pressure, and the clutch-actuating means actuates the clutch by using the accumulator pressure thus increased, whereby the other pair of wheels and the electric motor are connected to or disconnected from each other. As described above, by driving the pump-driving means, the accumulator pressure can be held such that it can engage or disengage the clutch, irrespective of a traveling state of the vehicle, and hence it is possible to engage or disengage the clutch at any time promptly and reliably. As a result, when it is required to drive the other pair of wheels by the electric motor, it is possible to drive them promptly and reliably, thereby ensuring a sufficient total driving force for driving the vehicle and enhancing of the marketability of the vehicle.
Preferably, the control system further comprises start-detecting means for detecting a start of the engine, and the clutch-actuating means engages the clutch when the start of the engine is detected by the start-detecting means.
According to this preferred embodiment, when the start of the engine is detected, the accumulator pressure is supplied to the clutch by the clutch-actuating means to thereby engage the clutch. In this process, the accumulator pressure is held by the control means such that the accumulator pressure can engage and disengage the clutch, so that when the vehicle is started, the clutch can be engaged promptly and reliably, whereby the other pair of wheels can be driven promptly and reliably by the electric motor. This makes it possible to ensure an excellent startability of the vehicle even on roads having small surface resistances, thereby further enhancing the marketability of the vehicle.
Preferably, the pump is an electrically driven pump, and the pump-driving means comprises an electric motor provided separately from the electric motor for driving the other pair.
Alternatively, the pump is a mechanically driven pump, and the pump-driving means comprises means for mechanically connecting between the mechanically drive pump and the electric motor for driving the other pair.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.